1. Field of the Invention
This invention relates to RF amplifiers and, more particularly, to a spread spectrum pilot signal extraction scheme for feed-forward RF amplifiers.
2. Description of Related Art
Feed-forward correction is routinely deployed in modem amplifiers to improve amplifier linearity with various input patterns. The essence of the feed-forward correction is to manipulate intermodulation (IMD) components created by the amplifier so that at the final summing point, the IMD components from the correction path cancel out the IMD components of the RF signal on the main path. Due to the unpredictability of input RF carrier pattern as well as the resultant IMD component location, a known frequency component, i.e. the pilot, is injected in the main loop to mimic the IMD components produced by the amplification process. By designing control circuitry to detect and cancel the pilot, the IMD components can also be removed. A common feed-forward correction scheme attempts to remove the IMD components by injecting a continuous wave (CW) frequency pilot into the RF signal. The CW pilot feed-forward correction scheme, however, requires a relatively sophisticated algorithm to constantly search for a relatively quiet location in the spectrum to place the pilot. The scheme also requires an accurate, synchronous power detector to determine the power of the pilot at the output coupler. The amplitude of the pilot at the output should not be higher than any of the IMD components. Otherwise, the pilot stands out as an unwanted distortion product like the rest of the IMD components. Consequently, the dynamic range of this scheme is limited because if the level of the CW pilot is too high, it interferes with the carriers.
Spreading a CW frequency pilot to produce a spread spectrum (SS) pilot simplifies the cancellation control circuitry and extends the pilot cancellation dynamic range. The spread spectrum pilot is spread over a wider bandwidth than the CW pilot, thereby reducing the average amplitude of the pilot at any single frequency within the wider bandwidth. When the power level of the SS pilot needs to be detected after being coupled off the output path, the SS pilot can be de-spread to extract the original CW frequency pilot. As such, the pilot is easier to detect because the SS pilot is de-spread and thereby has a much higher amplitude than the surrounding carriers and IMD components which were spread out during the despreading of the SS pilot. As such, IMD cancellation is enhanced because the pilot level can be more accurately monitored over a wider dynamic range. Because the SS pilot is spread over a wide band, there is no requirement for precise tuning to a "quiet" place in the spectrum. One drawback of the conventional spread spectrum pilot recovery scheme is that while the pilot gets de-spread, the rest of the frequency spectrum, such as the RF carriers and the IMD components, is spread, thereby raising the noise floor. The spread RF carriers (energy-rich) and the IMD components raise the noise floor which effectively reduces the detection range of the de-spread pilot.
Thus, a need exists for a pilot extraction scheme which reduces the drawbacks associated with current schemes.